1. Field of the Invention
This invention relates generally to nuclear magnetic resonance (NMR) apparatus and techniques for logging wells. More specifically, the invention relates to antenna designs for NMR well logging apparatus.
2. Background Art
Nuclear magnetic resonance (NMR) logging tools measure the amplitude and the decay constant of an NMR signal from the spin nuclei in earth formation, most often protons that are constituents of both water and hydrocarbons. The initial signal amplitude is a measure of total formation porosity while the time decay, invariably multi-exponential, can be decomposed into a distribution of exponential decays with different transverse relaxation times. The transverse relaxation time, T2, is a measure of spin-spin interaction that provides information on the pore size, type of fluid, and hydraulic permeability of the formation. These parameters are important petrophysical quantities, explaining why NMR logging is popular.
The quality of NMR logs is strongly dependent on the signal to noise ratio, S/N, of the measurement. S/N is determined by, among other parameters, the strength of the static magnetic field, the strength of the RF field, and the relative orientation of these two fields in the sensed region. The S/N also depends on the volume of the sensed region. In pulse NMR logging tools, a static magnetic field, B0, along the z-axis, is used to polarize the nuclear spins, causing the individual spins to precess around B0 at the so called Larmor frequency, xcfx89 L. In a typical measurement cycle, the RF field, B1, is used to flip the magnetization to another plane, often perpendicular to the direction of static magnetic filed, to generate an NMR signal in the receiving antenna.
The RF antennas used in NMR instruments can be approximated as magnetic dipoles. NMR logging instruments utilize an inside out design where the sample, a region of an earth formation for example, is outside the NMR instrument. The design of antenna for such applications is demanding. Two existing commercial NMR logging tools use shorted half coax and loop designs.
The RF antennas for NMR logging applications normally operate in the low MHz frequency range where the wavelength is on the order of 100s of meters. The physical length of these antennas is less than a meter and the operating depth from the borehole (DOI) is on the order of centimeters, thus the antenna always operated in the near field condition. This is an important distinction since all the antennas proposed or used in NMR logging are equivalent to simple dipole antennas in the far field, but they have much more complicated near field behavior.
Pulsed NMR instruments contain means of generating a DC magnetic field, B0, for aligning the nuclear spins, thereby producing a nuclear magnetization that initially is along the direction of B0. In addition an antenna is used for generating RF pulses, B1, that manipulate the nuclear magnetization and lead to measurable NMR signals. In principle, S/N is at a maximum when the orientation of B0 and B1 are orthogonal.
One of the widely used antennas in the field of communications is the simple loop antenna that is usually made with circular or square shape. A variant of this antenna that is used in NMR applications is the surface loop (SL) antenna. The name stems from the fact that loop is placed on, and takes the curvature of, the sensor surface. This makes the antenna as close as possible to the sample (earth formation). To further enhance the efficiency, these antennas are preferably placed on the surface of a material having high magnetic permeability, such as soft ferrites or other non-conductive material.
Other antenna enhancements have included multi-turn antennas or antennas having multiple windings to increase the total RF field generated by the antenna. With each additional turn, the RF field from that winding is approximately summed with the RF field from parallel windings. However, the efficiency of the coil is proportional to the amount of electric current flowing through the windings. As the number of turns increases, the impedance of the coil increases, which, for the same voltage source, leads to a lower current through the coil. Thus, for antennas having a good impedance match with the voltage source, additional turns alone does not lead to any B1 enhancement. Even if increased impedance improves antenna matching, such as one improperly matched at the outset, the antenna enhancement is seen across the entirety of the radiating surface of the antenna coil. Since, for NMR purposes, only the component of B1 that is orthogonal or nearly orthogonal to B0 is preferred, enhancing B1 at all points of the coil is not the most efficient approach.
An antenna coil in a downhole NMR measurement device includes at least a first coil portion that contributes to the total RF field generated by the antenna coil. Portions of the coil are enhanced so that the enhanced coil portions contribute a higher proportional contribution to the total RF field than the unenhanced coil portion.
According to an embodiment, the antenna enhancement is achieved by providing additional windings at or near the portion of the antenna coil that is to be enhanced.
Another enhanced antenna design utilizes an enlarged portion of the antenna coil in place of the additional windings.